A trackball assembly has a ball for controlling the position of a pointer on a screen of a computer. Rotation of the ball by the user is transferred into two orthogonal directions such as X and Y coordinate directions, for example, to move the pointer on the computer screen in the X and Y directions. Transfer of rotation of the ball into the X and Y directions is obtained through having two rollers, which are perpendicular to each other, engaging the ball. Each of U.S. Pat. No. 4,562,347 to Hovey et al and U.S. Pat. No. 5,008,528 to Duchon discloses each of two perpendicular rollers being resiliently biased by a separate spring into engagement with the ball of the trackball assembly.
One problem with the spring loaded rollers is that it is difficult to manufacture relatively small springs with the same loading force. When the spring forces are not equal, the rollers will not always correctly sense the amount of movement in each of the X and Y directions. This is because a different frictional force will exist between each of the rollers and the ball whereby one of the rollers will turn more in response to the same amount of ball rotation in each of the X and Y directions than the other of the rollers. When rotation of the ball produces movement in only one of the X and Y directions, the roller with the higher spring force may not slip as required; thus, the roller would incorrectly indicate there is movement in the direction in which it is sensing when there is not.
Another problem with having a spring exert a force on each of the rollers is that the spring can create a substantial force on the roller. This causes wear of the elastomeric material of the roller. It also can cause the elastomeric material to take a set resulting in a flat spot on the roller.
A further problem with the use of a spring to load each of the rollers is that there is a limit as to reducing the size of a spring while still producing a desired force. Thus, miniaturization of the trackball assembly cannot be accomplished where space is limited.
Another problem with a trackball assembly is that the ball will not float freely within defined constraints to allow self alignment of the ball within its retaining cap or cover. Large tolerances resulting in building the overall trackball assembly prevent the ball from centering within the cap or cover and cause the ball to bind. Twisting of the trackball assembly by mounting forces can also cause the ball to bind.
One previous arrangement to eliminate binding of the ball has been to use a spring loaded cap or cover. However, the spring loaded cap or cover is expensive and produces non-uniform forces on the ball in multiple directions so as to cause drag on the ball. Additionally, with the spring loaded cap in which a leaf spring is mounted in the cap to act on the ball, miniaturization of the trackball assembly cannot be obtained practically or inexpensively. Another prior arrangement to eliminate binding of the ball has been to incorporate expensive machining operations to reduce the tolerance build up to facilitate centering of the ball within the cap or cover.